This invention generally relates to a fabrication method and product of a lightweight structural panel, in particular, a panel that is a laminate of metal face sheets and a plastic core body.
Lightweight structural panels are used in the construction industry, and are particularly desirable for outdoor use. The ideal panel is lightweight and relatively inexpensive to manufacture, has good insulation value, and is weather resistant. Panels that are durable, weather-resistant, and have high compressive and shear strength are particularly useful for outdoor applications. Structural panels having metal face sheets laminated with a plywood or wood composite core have been used, but are known to have the problem of being vulnerable to damage from moisture which can seep into the core. The moisture can cause the plywood or composite core to swell or rot. It is therefore preferable to replace the plywood or wood composite cores in structural panels for outdoor use.
Structural panels with metal face sheets bonded to a core of foam or a solid polyethylene sheet have also been used. Such a laminate can be formed with good adhesive bonding using an improved sheet bonding process developed by the present inventor and described in U.S. Pat. No. 5,114,510, entitled xe2x80x9cMethod of Forming a Lightweight Structural Panelxe2x80x9d, issued on May 19, 1992. While metal-faced panels with foam cores have been used successfully, they have insufficient shear strength for some high-load applications. Also, moisture collected in the foam core that is subjected to cycles of freezing and thawing can damage the panels by causing the layers to delaminate. On the other hand, use of a solid plastic core sheet is costly and wastes material in excess of what is necessary to provide adequate compressive and shear strength.
Honeycomb cores formed with contiguous hexagonal cells laminated between outer face sheets have also been used. However, the fabrication costs of honeycomb cores are relatively high. Therefore, honeycomb-core laminates, for example, as sold by Unida Corp. and Ciba Geigy Corp., are typically used in aircraft and other high-priced constructions, but are too costly for widespread use as construction material. The honeycombs cells are also aligned perpendicular to the face sheets, and thus offer very thin surface areas for bonding to the face sheets. This is a disadvantage since very careful gluing procedures and expensive high-strength adhesives are required. The cells are also closed off by the face sheets, thereby forming trapped air or moisture pockets which can generate expansion pressures from heat or freezing water that can delaminate the face sheets from the cores.
A structural panel formed of plastic face sheets fusion bonded to a corrugated plastic core sheet has also been used, for example, as described in U.S. Pat. Nos. 3,837,973 and 3,999,928 to Asakura et al. However, this type of corrugated all-plastic laminate does not have sufficient rigidity for many construction applications. Another type of structural panel has plastic face sheets bonded to a foam core which can retain its shape despite changes in temperature and humidity, for example, as described in U.S. Pat. No. 4,133,930 of the present inventor. While such plastic foam-core panels are resistant to weathering, they do not have sufficient shear strength for many construction applications.
Accordingly, it is a principal object of the invention to provide a lightweight structural panel that is resistant to swelling from moisture, weathering, and freezing and thawing cycles, is easy to fabricate and makes optimum use of materials so as to obtain the highest utility for the lowest material and fabrication costs, and at the same time has good rigidity and compressive and shear strength. In particular, it is desired to produce a structural panel that can survive the rigors of outdoor exposure and wide temperature changes, and meet the stress and load requirements for architectural and industrial use.
In accordance with the present invention, a structural panel has at least one face sheet made of a durable material having a high tensile strength, and a core body having two outer plastic liner sheets fusion-bonded to linear ends of inner plastic truss elements spaced apart with open spaces therebetween extending in a plane direction of the plastic liner sheets, wherein one of the outer plastic liner sheets of the core body is bonded with an adhesive layer to an inner surface of the at least one face sheet.
The adhesive bonding of the plastic liner sheets of the core body to the face sheet(s) provides a high-strength laminate bonding that is very resistant to delamination. The use of sheet-to-sheet bonding with an adhesive layer is simple and inexpensive. The all-plastic core body is highly resistant to moisture attack, and the open spaces between truss elements provide flexibility to accommodate thermal expansion and contraction. The use of a core body made of plastic truss elements fusion-bonded to plastic liner sheets provides high compressive and shear strength, while minimizing the amount of material required to be used, thereby keeping the cost low. The fusion-bonding of plastic face sheets and truss elements can be accomplished readily with conventional plastic extrusion techniques.
Metal face sheets are preferred because of their high tensile strength, although plastic and fiber-reinforced skins may also be used for specific applications. A preferred core body has the two plastic liner sheets fusion-bonded to the convex surfaces of a corrugated plastic core sheet formed by a conventional plastic extrusion technique. Use of the corrugated plastic core sheet for the truss elements provides the best combination of high strength, ease of fabrication, and low material cost. However, diagonal plastic webs or perpendicular plastic I-beams may also be used for the truss elements.
In a preferred embodiment, two metal face sheets are bonded adhesively to the outer sides of the plastic core body. In another preferred embodiment, two metal face sheets are bonded to two plastic core bodies which in turn are bonded adhesively to a center foam core layer. Yet another embodiment has one metal face sheet bonded to an outer side of a plastic core body, allowing it to be used as veneer or curved around rounded structures.
Other objects, features and advantages of the present invention are described in detail below in conjunction with the drawings, as follows: